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Seawater chemistry and global climate are intricately linked through various feedback processes that manifest, and are susceptible to change, on modern to geologic timescales. However, our understanding of modern biogeochemical processes in the ocean often falls short of explaining changes in seawater chemistry in the past. This deficiency may be attributable to global datasets that are used to balance oceanic geochemical budgets lacking coverage that includes convergent margin settings. Subduction zones are complex hydrogeologic and diagenetic environments that have the potential to disproportionately influence geochemical budgets on various timescales, with the geochemical composition of pore fluids and sediment serving as promising media to both trace the sources and migration of fluids in convergent margin complexes and reconstructing oceanographic and environmental change over time. This dissertation focuses on the south-central Chilean Margin, which is relatively understudied from a geochemical perspective when compared with other prominent subduction zones but has nonetheless been a target for oceanographic coring campaigns for the better part of three decades. Recent implementation of D/V JOIDES Resolution Expedition 379T (JR100) in Summer 2019 recovered sediment cores from across the Chilean Margin that document unique geochemical and lithologic features in the upper ~100 m of the sediment column, including marked reductions in dissolved chloride concentrations in pore waters, spatially heterogenic methane hydrate and authigenic carbonate accumulation, and cyclical variations in sediment coloration. This thesis utilizes geochemical (elemental, isotopic, radiogenic) measurements in new high-resolution pore water and bulk sediment samples to unravel the cause of these features and their potential connection with regional or global seawater chemistry, hydrogeologic processes in the accretionary prism, and climatic change over time. Chapter 1 demonstrates that pore fluid freshening in a core offshore Patagonia is attributable to the infiltration of fossil meteoric groundwater that has mixed with geothermal groundwater at depth. A comparison of new pore water chemical results with terrestrial fluid data highlights, for the first time, a hydrogeologic connection between Patagonian groundwaters and marine sediments offshore on the Chilean Margin. Additional details on methane hydrate dissociation and characterizations of the regional geothermal reservoir are also provided in this chapter.Chapter 2 addresses spatial heterogeneities in pore water chemistry and methane hydrate accumulation near a seafloor mound venting structure in the central sector of the margin. New elemental and isotopic measurements reveal deeply sourced fluids from mineral dehydration as the cause of freshening at the site closest to the vent structure but the diminishing freshening signal away from this feature indicates focused migration of deep fluids and gas may be a primary control on the spatially variable pore water chemistry and methane hydrate formation. Chapter 3 further assess pore water chemistry on the Chilean Margin. By compiling new data from JR100 and legacy data from prior Ocean Drilling Program expeditions, this work provides the first comprehensive characterization of pore water geochemistry on the Chilean Margin. We use new isotopic measurements to constrain fluid sources and migration pathways, which were previously unaddressed by shipboard measurements from prior expeditions. We also assess substantial spatial variability in the proximity of sites with and without pore water freshening, utilize deep Ocean Drilling Program cores to provide context for JR100 cores, and document an apparent lack of evidence for marine silicate weathering in these anoxic sediments. Lastly, Chapter 4 addresses the cyclical variations in greenish and bluish sediment coloration documented at sites along the Chilean Margin, quantified at the green-blue ratio. We demonstrate that the green-blue ratio is a measurement of biogenic opal (diatom) productivity, with high ratios reflecting relatively more diatom productivity over time, and vice versa. We calibrate the downcore record to generate the first continuous, high-resolution record of diatom productivity in the South Pacific over the last 150,000 years, which provides unprecedented constrains on biogeochemical-climate interactions in the late Quaternary.Collectively, this thesis makes important advances in our understanding of the hydrogeologic and diagenetic processes that characterize the south Chilean Margin, while also developing new tools to reconstruct environmental change in this region. Ultimately, the work presented here demonstrates that there is still much work to be done and questions to be addressed with continued research on the Chilean Margin. |
